Now that David Letterman has extended his deal with CBS through 2015, it's time to ask: How long can the onetime heir to Johnny Carson stay on the job?

Letterman, host of CBS' "Late Show," will turn 68 in 2015. Television executives strive to reach "the demo," or the demographic comprised of viewers ages 18 to 49. At 68, Letterman wouldn't just be outside the demo, he'd be at least as old as the fathers of most people inside it. As painful as it is for those of us who grew up on his old NBC late-night show to admit, this guy is your dad's TV host — at least if you're under 40.

Is that a horribly ageist thing to say? Maybe. But it's also realistic. Before you bury me under a mountain of comments and emails for being a hateful person who wants to rob an old pro of a steady gig, remember that Letterman has faced succession speculation for years, dating back at least to 2000, when he underwent heart surgery and missed five weeks of work.

Nothing lasts forever, and television hosting gigs, just like TV hosts themselves, are no exception. Yes, I know that CBS is comfortable with older viewers and that Andy Rooney kept working on "60 Minutes" until a month before he died at age 92. But hosting a nightly talk show is different. It's a young person's game. There's a reason why NBC is finally putting Jay Leno to pasture (at age 63) and replacing him with Jimmy Fallon (39). Look at the number of jokes — many of them from Letterman! — about Larry King's age during his final years on CNN. (King's marriage to a woman 26 years his junior also made him a target, but that's a different issue.)

"Late Show" has never reached the heights of absurdity Letterman hit on his old NBC show, which had the freedom of a relatively expectation-free 12:30 a.m. time slot. Letterman became a sensation during an era when most people still watched just three networks and late-night competition was virtually nonexistent. Now, while Letterman still knocks off some amazing interviews with celebrity guests and newsmakers, the comedy on the CBS show has settled into a calcified formula, with the humor a lot broader and less cerebral and a large band providing aggressive musical cues. (Some faint glimmers of the old surrealism remain, such as in the off-again, on-again bit "Is This Anything?") It's a recipe that still delivers, but alongside the likes of Jon Stewart and Stephen Colbert, it's also tasting a little stale. Which is ironic because 30 years ago Letterman's humor was so subversive and bracing many believed he'd never find work outside of the wee hours.

The ratings are revealing. During premiere week this fall, Letterman was No. 2 to Leno's "Tonight Show," with 3.1 million total viewers versus 3.7 million, according to Nielsen. But among adults ages 18 to 49, "Late Show," which just marked its 20th anniversary on CBS, ranked third behind Leno and ABC's Jimmy Kimmel. In fact, Letterman had the same 18-to-49 rating as NBC's "Late Night With Jimmy Fallon" show, despite the fact that Fallon airs an hour later.

So how long will Letterman hold out? Will his dotage be televised, or did we just witness the last re-upping for the man many believe is the best talent in late night?

Time will tell, but we already know one thing: Letterman has surpassed his idol Carson in one notable respect.

Carson, the undisputed King of Late Night who laid down the rules everyone else has either followed or broken for the last 50 years, left "The Tonight Show" in May 1992.